The evolving complexity of infrastructure often requires precise location and identification of utility lines (e.g., underground power lines, gas lines, phone lines, fiber optic cable conduits, cable television (CATV) cables, sprinkler control wiring, water pipes, sewer pipes, etc.) for purposes of repair, enhancement, and/or replacement. Such utility lines, collectively and individually referred to herein as “buried objects” or “buried utilities,” may be buried in the ground and/or otherwise hidden from normal sight. Construction and/or excavation operations typically require the locations and/or identification of such utility lines be known so as to avoid costly and hazardous destruction of infrastructure (for example, so that a buried natural gas line is not ruptured during excavation of the ground for work on other utilities).
In utility locating operations (also denoted as “locates” for brevity), one or more locating devices, also referred to herein as “buried utility locators,” “utility locators,” or simply “locators” for brevity, may be carried and moved about a locate area to detect, process, and/or record magnetic field signals for use in determining information associated with the utilities and/or other conductors in the ground. For example, one or more locators may be moved over the ground or other surface by an operator, with each locator receiving magnetic field signals emitted from one or more utilities. In one or more processing elements of the locator, the magnetic field signals are then processed to determine information about the buried utility, such as its position relative to the ground surface, depth, type of utility, geographical location, and the like.
If the buried utilities are conductors that carry their own alternating current electrical signal, they can be traced by detecting emitted magnetic field signals at their correspondingly energized frequency or frequencies, such as 50 or 60 Hz, or harmonics thereof, from underground power cables. This is commonly known as “passive locating” (i.e., detecting magnetic field signals emitted from currents flowing in a utility due to a signal applied to the utility, as in the case of an electrical power signal, or induced in the utility from electromagnetic radiation from power lines, radio transmitters, or other signal sources).
Signals may also be coupled to the utility by a user during a locate operation. These signals have a predefined frequency or frequencies, and may be generated in a device known in the field as a locate transmitter or “transmitter” for short. The output of the transmitter may be directly, inductively, or capacitively coupled to the utility to induce current flow therein. This type of locating is commonly known as “active locating.”
In addition, in some locating operations, a device known as a sonde, which includes a magnetic dipole antenna and signal generation module, commonly powered by a battery, is inserted into a pipe, conduit, or other cavity and generates a dipole magnetic field signal that can be detected by the locator. Some locate operations use two or more of these locating techniques at a time, while others rely on a single emitted signal to determine buried utility information. Portable utility locators typically include one or more antennas that are used to detect the magnetic field signals emitted by buried pipes and cables and/or by sondes that have been inserted into pipes.
In addition to the above magnetic field signals, some underground utility installations use marker devices placed adjacent to the utilities. Such marker devices are typically passive markers including a single resonant circuit for operating in a resonance mode responsive to a signal transmitting electromagnetic energy at a specific frequency, which is expected to be re-transmitted at the same frequency for detection of such marker devices. These marker devices lack control over the received electromagnetic energy, which is often affected by its form factor, component construction, manufacturing tolerances, underground environment (e.g., wet or otherwise conductive soil) where the marker devices are placed, etc. This can negatively affect performance of such marker devices and result in an output signal (re-transmitted signal) having a gradually decayed amplitude often undetectable by a receiver (e.g., locator antenna) or detectable, occasionally, with a limited signal range requiring close coupling with the receiver. Further, re-transmission of electromagnetic energy, from the marker device to the receiver, at the same frequency or nearly same frequency of the received signal results in backscattering of the re-transmitted electromagnetic energy at the receiver, resulting in substantial interference making detection of the marker device difficult and/or erroneous. Additional and expensive devices/components are often required for an attempt to reduce backscattering.
Accordingly, there is a need in the art to address the above-described as well as other problems related to marker devices and associated locating systems.